Currently known metal adsorbents include a material having a functional group which adsorbs metal ions and which is chemically bonded to a carrier and a low-molecular-weight compound having such a functional group, which compound is enclosed by a carrier so as to prevent elution of the compound. Examples of such metal-adsorbing functional groups include a carboxyl group, a sulfonate group an amino group, an imino group, a thiol group, and a phosphoric group. Such metal-adsorbing functional groups exhibit preference to metal species which are readily adsorbed. For example, the sulfonate group predominantly adsorbs monovalent metal ions such as potassium and sodium, whereas the carboxyl group and the phosphoric group can adsorb a wide variety of metal ions, including calcium and magnesium. The amino, imino, and thiol groups are reported to adsorb heavy metal ions well. Thus, the metal-adsorbing functional group is appropriately selected in consideration of a target metal species to be adsorbed. Regarding the carrier, a variety of species are employed, and examples include vinyl resins such as styrene resin and acrylic resin, and natural materials such as cellulose (powder, fiber, and gel), chitin, chitosan, and wool. In a typical production process of metal adsorbents, a metal-adsorbing functional group is introduced into a polymerizable monomer, and then the monomer is polymerized, from the viewpoint of easiness of synthesis and uniformity of the products. The thus-produced polymers are known to be ion-exchange resin and chelate resin. In another production process, a metal-adsorbing functional group is chemically introduced into a natural material such as cellulose or wool from the viewpoint of low material and production costs.
Meanwhile, metal adsorption performance of metal adsorbents varies depending not only on the chemical structure of the material that constitutes the adsorbents but on the product form thereof also. Generally, resin-based materials are molded into beads by virtue of the moldability thereof. However, such beads products have problems in relation to water treatment efficiency in that a large number of functional groups are confined inside the beads; that rates of diffusion of metal ions and a regenerating agent into the resin decrease due to the hydrophobicity of resin; and that the minimum adsorption level increases.
The mode of employing a metal adsorbent is also limited by its product form. For example, in the case of removal of metals contained in water, a metal adsorbent which is produced through graft polymerization of a resin material or a material chopped into small pieces with a metal-adsorbing functional group can be employed by feeding the metal adsorbent into water so as to adsorb metals and recovering the metal adsorbent through centrifugation or filtration. Alternatively, the above metal adsorbent is employed in a column. When the particle size of the above metal adsorbent is small, high-speed and high-efficiency filtration is impeded, which is not suited for treatment of a large amount of water.
When treatment for removing or recovering metals, particularly softening of water, removal of hazardous metals from industrial wastewater, removal of hazardous metals from polluted soil, or recovery of useful metals, is performed, the volume- and the area-to-be-treated becomes large, and thus a large amount of metal adsorbent is required Thus, preferably, the metal adsorbent for the above uses exhibits high adsorption performance and is inexpensive and regenerative
The phosphoric group, serving as a metal-adsorbing functional group, has the following characteristic features: 1) one single phosphoric group can adsorb a divalent metal ion, thereby attaining high metal adsorption performance, 2) the phosphoric group readily releases a hydrogen ion in an acidic pH region, whereby metals contained in a solution can be adsorbed within a wide pH range; and 3) the minimum metal ion adsorption level is low. Cellulose, serving as a carrier, has the following characteristic features: 1) the fibrous structure of cellulose provides high rigidity; 2) a large number of functional groups are exposed to the fiber surface; and 3) high processability.
In consideration of the above characteristic features, metal adsorbents including a carrier made of natural material and a phosphoric ester group have drawn attention. Hitherto, there have been known such products and techniques in relation thereto including cellulose phosphoric ester for use in removal of heavy metals and radioactive metals (Patent Document 1); cellulose phosphoric ester having high mechanical strength produced by employing sulfur powder in the production of the ester (Patent Document 2): phosphoric, acetic, or benzoic esters of cellulose and starch for use in removal of heavy metals from water (Patent Document 3), and a filter made of cellulose having a carbamide group and a phosphoric ester group for use in removal of hardening components and heavy metals from drinking water (Patent Documents 3 and 4) However, these products are not satisfactory in terms of metal adsorption performance and economy. Therefore, keen demand has arisen for a low-cost metal adsorbent which exhibits higher metal adsorption performance, higher rate of reaction with metals, higher mechanical strength, higher processability, higher adaptability, higher reusability, etc.    Patent Document 1: Russian Patent No. 2096082 C1    Patent Document 2: WO 99/28372    Patent Document 3: German Patent Application Laid-Open No. 19859746 A    Patent Document 4: Japanese kohyo Patent Publication No. 2003-500199